Newly developed cloak hides underwater objects from sonar

Jan 05, 2011

Illinois researchers designed a two-dimensional cylindrical cloak made of 16 concentric rings of acoustic circuits structured to guide sound waves. Each ring has a different index of refraction, meaning that sound waves vary their speed from the outer rings to the inner ones. Credit: L. Brian Stauffer

In one University of Illinois lab, invisibility is a matter of now you hear it, now you don't.

Led by mechanical science and engineering professor Nicholas Fang, Illinois researchers have demonstrated an acoustic cloak, a technology that renders underwater objects invisible to sonar and other ultrasound waves.

"We are not talking about science fiction. We are talking about controlling sound waves by bending and twisting them in a designer space," said Fang, who also is affiliated with the Beckman Institute for Advanced Science and Technology. "This is certainly not some trick Harry Potter is playing with."

While materials that can wrap sound around an object rather than reflecting or absorbing it have been theoretically possible for a few years, realization of the concept has been a challenge. In a paper accepted for publication in the journal Physical Review Letters, Fang's team describe their working prototype, capable of hiding an object from a broad range of sound waves.

The cloak is made of metamaterial, a class of artificial materials that have enhanced properties as a result of their carefully engineered structure. Fang's team designed a two-dimensional cylindrical cloak made of 16 concentric rings of acoustic circuits structured to guide sound waves. Each ring has a different index of refraction, meaning that sound waves vary their speed from the outer rings to the inner ones.

"Basically what you are looking at is an array of cavities that are connected by channels. The sound is going to propagate inside those channels, and the cavities are designed to slow the waves down," Fang said. "As you go further inside the rings, sound waves gain faster and faster speed."

Illinois professor Nick Fang developed a two-dimensional acoustic cloak that makes objects in the center invisible to sonar and other ultrasound waves. Credit: L. Brian Stauffer

Since speeding up requires energy, the sound waves instead propagate around the cloak's outer rings, guided by the channels in the circuits. The specially structured acoustic circuits actually bend the sound waves to wrap them around the outer layers of the cloak.

The researchers tested their cloak's ability to hide a steel cylinder. They submerged the cylinder in a tank with an ultrasound source on one side and a sensor array on the other, then placed the cylinder inside the cloak and watched it disappear from their sonar.

Curious to see if the hidden object's structure played a role in the cloaking phenomenon, the researchers conducted trials with other objects of various shapes and densities.

"The structure of what you're trying to hide doesn't matter," Fang said. "The effect is similar. After we placed the cloaked structure around the object we wanted to hide, the scattering or shadow effect was greatly reduced."

An advantage of the acoustic cloak is its ability to cover a broad range of sound wavelengths. The cloak offers acoustic invisibility to ultrasound waves from 40 to 80 KHz, although with modification could theoretically be tuned to cover tens of megahertz.

"This is not just a single wavelength effect. You don't have an invisible cloak that's showing up just by switching the frequencies slightly," Fang said. "The geometry is not theoretically scaled with wavelengths. The nice thing about the circuit element approach is that you can scale the channels down while maintaining the same wave propagation technology."

Next, the researchers plan to explore how the cloaking technology could influence applications from military stealth to soundproofing to health care. For example, ultrasound and other acoustic imaging techniques are common in medical practice, but many things in the body can cause interference and mar the image. A metamaterial bandage or shield could effectively hide a troublesome area so the scanner could focus on the region of interest.

The cloaking technology also may affect nonlinear acoustic phenomena. One problem plaguing fast-moving underwater objects is cavitation, or the formation and implosion of bubbles. Fang and his group believe that they could harness their cloak's abilities to balance energy in cavitation-causing areas, such as the vortex around a propeller.

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A pen can stir the heart or stab it. Clearly we're OK with the concept of abused tools. That this'll change the existing power balance is as mundane as saying computers will change the existing power balance.

The important consequence of this research is that submarines will become even important in naval operations, assuming the cost of these cloaks is not prohibitive. There are more subs today than during the Cold War. Of course bad guys will do bad things with technology. Should we all go back to the cave and die of old age at 40?

"Ethics" and/or "Morality" with regard to scientific undertakings -- how quaint. The time for that has long since past. Hopefully before too long Homo Destructus will have "extincted" themselves and it will be a moot question.

Really. In the near future, this meta material cloak may be too expensive for naval vessels. But it shouldn't be too difficult to implement it on small submarine drones. These drones would be undetectable even with an active sonar.

In case you didn't know: If you are reading about a NEW stealth technology that is being openly printed in the public media you can be sure that what you are reading is PROPAGANDA!Here is the logic: If a new stealth technology is being desribed TO THE PUBLIC it is because it is:a)Impractical, and/or cost prohibitive, and they already know how to counter it!(because if it doesn't work or is too expensive, and they already know how to counter it, their hope would be by "spilling the beans" that enemies are hopefully stupid enough to waste their money on it while they have nothing to fear from it).or,b)It doesn't really exist.(then the propaganda is to make enemies think you're ALL SEEing, INvisible, INvincible, and ALL POWERFUL and also to make your own sheeple think that they have the MOST badass military EVER so that they will fear less and are willing to pay more taxes to keep it that way. Do you really believe that you ALL got magically upgraded to PRIVILEGED information?

We definitely have rail guns and lasers on our ships, and some aircraft now have laser. Not guidance lasers, but "directed energy weapon" class lasers called COIL, THEL and others. These things definitely exist and have been publicly demonstrated.

The best thing about having the best military ever is that no rational enemy would ever want to fight us, particularly not in naval or aerial combat.

For example, stealth is a tactical advantage that is soon lost when knowledge of its workings gets out. Since there is nothing stealthy about a railgun mounted to a gigantic ship, no tactical advantage is lost by people knowing about it. knowledge in this case serves as the deterent and therefore serves to our advantage.

If you spend billions developing stealth tech and further billions fielding it YOU KEEP ITS WORKINGS SECRET in order to protect your investment. Leting out practical stealth technology to the public undermines its very purpose. I'm not saying you don't brag about stealth, but you certainly don't describe exactly how it works in a public setting.

Really. In the near future, this meta material cloak may be too expensive for naval vessels. But it shouldn't be too difficult to implement it on small submarine drones. These drones would be undetectable even with an active sonar.

This could render current naval vessels completely obsolete.

Nah. It would be supplemental. You'd still have existing subs as both a local command hub and sensors. you'd have small un-manned subs armed with torpedoes for sneaking up on enemy positions and attacking. It would work just like UAV drones in the air.

The best military ever, ha ha ha. You mean the most expensive, farmers can defeat your steroid addled war criminals. Man the hubris of the uneducated never ceases to amuse.

What never ceases to amuse me is the blatant idiocy apparent in statements (like the above) made by people who claim to have not only an education, but apparently assert they have a high level of education...

@ ilbud: What does this have to do with stealth underwater technology? If you claim to be "of the educated", perhaps you should stay on topic in a forum that tries its best to be scientific

A point the article makes is that the technology could be used to minimize cavitation, which might do more for making a sub stealthy than covering the entire sub in this material could. The tactical advantage is not in merely having the technology. The advantage comes in effectively using it.

The best military ever, ha ha ha. You mean the most expensive, farmers can defeat your steroid addled war criminals. Man the hubris of the uneducated never ceases to amuse.

You uh, realize how one-sided the Iraq invasion really was? The Iraqi pilots knew they had no chance, so they didn't even bother to try to fight back.

the majority of U.S. and coalition troop casualties have happened because of our incompetent government's insistence on fighting terrorists "man to man" in remote villages in Afghanistan, combined with the completely absurd "rules of engagement" which our military has adopted.

If we adopted "Real Time Strategy game" style rules of engagement, we'd level the whole place in an hour or two and go home with almost no casualties.

We are 10 - 15 years away from total obsolescence of anything that flies in the sky, or re-enters from orbit. That means our nuclear delivery systems will be nearly useless. Directed energy weapons are here, and their power factors will reach the point where nothing can survive within their range. Armor, aircraft, bombs, and missiles will not be as dependable in the new century as it was in the old.

Meanwhile, China has been given nearly all the manufacturing of our economy, which means their technical acumen will increase while ours atrophies.

Their empire will rise, and ours will diminish, and our dependency on them (arranged by traitorous and foolish American greed) will create a dangerous security condition for us and our allies.

It's a shame that we are so ignorant of the consequences of our actions. Poor government precipitates collapse, and collapse precipitates civil war. We are not immune from this historical constant in the USA.